Mechanism for operating a load limit device in response to excessive load on a driving member



LOGIC! Lbs.

July 15, 1958 w. G. CHAMPION ETAL 2,843,691 MECHANISM FOR OPERATING A LOAD bIMIT DEVICE IN RESPONSE TO ExcEssIvE LOAD ON A DRIVING MEMBER Filed Oct. 20, 1955 IN V EN TORS' Q. Gmmpz'or Thomas Barns-) Pl/endcll 5 5 I0 8 so do .90 10a Dcfleciiorz Tlzoasandilzs/ 17c MEHANEM IFQR UPERATHNG A LOAD Lil /lil" DEVICE- iN REEFGNSE TU EXCESSIVE LOAD N A DRlViNG MEMBER Wendell (G, (Jhampion, Grand Rapids, Mich, and Thomas liarish, Shaker Heights, Ulric, assignors to Lear, incur porated, Grand Rapids, Mich, a corporation of Illinois Application @ctoherZli, 1955, Serial No. 541,647

1 Claim. (Cl. 200-6146) This invention relates to improvements in means for actuating electrical limit switches or other load limiting devices pursuant to overload on a driving member.

it is common to utilize a switch for interrupting current to an electric motor when a pre-determined load on the mechanism driven thereby has been exceeded. Where a reversible mechanism is involved a pair of switches or one switch operable in both directions is employed. In either case it is also sometimes customary to arrange for automatic re-closing of the circuit when the excessive load has been alleviated and, for this purpose, bias means are utilized for normally maintaining the switch or switches in circuit-closing positions.

For example, in United States Patents Nos. 2,403,102 (owned commonly with the present application) and 2,222,716 there are disclosed arrangements foreiiecting the broad purpose just stated. in both prior disclosures overload is sensed through the ring gear of a planetary transmission which gear is mounted for limited rotation in either direction in accordance with the direction of the overload torque. An arm protruding radially from the circumference of the ring gear or means effectively a radial arm thereof is biased to a central position by compression coil springs of uniform diameter, Upon displacement of the arm due to overload one or' the other of a pair of limit switches is opened and will be reclosed by its pertinent bias spring when the overload is removed. It is common to provide for adjustment of the bias springs in accordance with the overload at which circuit interruption is required.

In order to provide an adjustable device of the foregoing general character it has been found that accurate relationship between the magnitude of the predetermined overload and the deflection of the bias springs must be maintained. The springs must possess the ability todeflect only negligibly as the load approaches the pre=set maximum and then, at the instant this load is reached considerable deflection with small increase in load is the desideratum. The foregoing characteristics render it possible to utilize over-center, snap-action switches (sometimes loosely designated by the trademark Micro- Switch). These switches are extremely reliable, particularly in aircraft applications as they can, due to the low inertia of their parts, easily be rendered insensitive to acceleration forces. However, they are desirably operated upon application of a low order of positive pressure exerted over an extremely short stroke. Thus, if the movement transmitted from the ring gear, for ex ample, is exerted against biasing springs of the type illustrated in the prior art cited above, and over-center, snapaction switches are employed, Hookes law governs and the desired spring response outlined above is impossible to obtain.

The present invention relates to an improved form of bias spring which will provide the desired response in the particular environment outlined hereinabove; namely small displacement initially and large displacement subgram Patented July 15, 1958 sequently based on a uniformly increasing load, i. e. force on the spring.

in another aspect the invention is concerned with translating displacement of the spring into a smaller displace-- ment for actuation of so-called Micro-Switches which, as has been noted, operate more efficiently on smaller movements of the operating button or plunger. Mechanical linkages for this purpose are contraindicated as the backlash is likely to constitute asubstantial portion of the plunger stroke and erratic operation occurs. Moreover, a direct connection, such as a cam, must take account of possible injury to the switch plunger due to friction or jamming.

Consequently, another object of the invention resides in providing a resilient connection between the bias means and switches which will insure reliable operation thereof without danger of damaging the same and also provide a desired ratio between displacement of the spring andthe stroke of the switch plunger.

Other objects will become apparent from the following description which, taken in conjunction with the accompanying drawing will illustrate certain preferred modes in which the invention may be embodied.

In this drawing:

Fig. 1 is an isometric view of an arrangement incorporating the invention;

Fig. 2 is an isometric detail of one. form of bias spring in accordance with the invention, another form being included in Fig. 1;

Fig. 3 is an isometric detail of a switch actuating element; and

Fig. 4 is a graph illustrating the characteristics of the invention spring as compared to those of a coiledcompression spring of uniform diameter.

in this description reference is made, by way of example, to a planetary transmission, the ring gear of which is arranged for rotary displacement under overload and hence capable of actuating a limit switch or switches. However, it will be comprehended that the overload may be sensed under various other conditions of operation, both rotational and translational. Moreover, while two switches are shown the invention is equally applicable to only one. 2

Turning now to the drawing We have shown, by way of example, a load transmitting organization ll), in this case a planetary transmission including a sun gear 11, planet pinions l2 and a ring gear 13. A source of input power (not shown) is connected to the sun gear 11 as will he understood, and may be an electric motor, the supply to which is interruptedupon occurrence of overload.

Surrounding the ring gear is a hollow cylindrical member or sleeve 17 adapted to rotate with the ring gear 13 through the medium of a plurality of friction shoes 19. Operation of the overload arrangement of the invention is through torque exerted through the ring gear 13; the sleeve 17 and the friction shoes being provided to allow slippage in the event of overload beyond the permissible deflection of the bias springs. However, as will appear subsequently the ring gear may actuate the overload mechanism without the intermediation of the sleeve. Suitable support is provided to allow limited angular movement of the ring gear and sleeve or the ring gear alone, as the case may be. In either case overload to which the transmission it) may be subjected is evidenced as angular deviation from a normal position of an arm 21 extending radially from the sleeve 1'7.

Suitably supported on either side of the distal end of the arm 21 is a pair of springs 22 and 22a which may, for purposes of this specification, be described as J-shaped. More particularly these springs include a longer leg 24, a bight 25 preferably extending over an arc slightly greater than 180, and a shorter leg 26. The leg 24 may be straight, as in Fig. l, or provided with an arcuate portion 31, as in Fig. 2. In either case apertures 28 and 29 are provided for passage of a stud 33 upon which a nut 34 is threaded for adjustment of the spring tension. Studs 33 are mounted in a fixed bracket 35 to which the opposed flat faces whereof the leg as are juxtaposed. A head 41 at the distal end of the arm 21 is in operative abutment with both springs 22 and 22a and maintained at normal or center position thereby.

Obviously, since apertures 28 and 29 have some clearance on the studs 33, any adjustment or" the nuts is effective to change the force exerted by the springs on the arm 21 and it will be clear, moreover, that the two adjustments are independent. Desirably the other support for the springs 24 comprises a fixed rod or bar slidably receiving the slotted ends of the legs 24.

A pair of switches 51 and 51a are mounted in any suitable manner on the framework (not shown). These may be of the over-center, snap-action type characterized by a comparatively short stroke of the operating button 52 and low order of operating pressure. in order that the usually larger displacement of the spring leg 24- may be transformed into the usually smaller displacement of the button 52 we have introduced intermediate members 54 and 54a (Fig. 3) which include a mounting part 55 for securement to any convenient fixed location, e. g. the body of the switch and a resilient arm 56 having a laterally-extending finger 57. This latter element has an inclined surface 58 for camming actuation of the button 52. The parts of the member .lt are so proportioned and arranged as to translate maximum excursion of the leg 24 into the corresponding range of operation of the switch button 52. By virtue of the resilient construction of the arm 56 jamming action of the switch is obviated in the event that, due to some defeet in an organization including the invention, the circuit fails to open.

For normal operation the springs 22 and 22a will be so designed and adjusted as to maintain the arm 21 and hence the ring gear 113 immobile or at least suificiently so as to avoid operation of either limit switch 51 or 510. However, in the event the normal load transmitted to or through the transmission 10 is exceeded the torque thereof is effective to rotate the ring gear 13 in one direction or the other depending upon the then direction or" rotaticn of the sun gear 11. When this occurs the force of the spring 22 or 22a is overcome and the pertinent switch 51 or 51a is operated through a member 54 to open the circuit to the driving source whereupon the arm 21 is returned to its center position under the bias of the springs.

Basically both forms of spring shown in Figs. 1 and 2 provide the same response which is illustrated as curves A and B of Fig. 4 wherein load in pounds is plotted against deflection in 0.010" increments. It will be understood that the load will be that which corresponds to the torque setting measured on a lever arm from the point of abutment of the head 41 with the springs to the axis of rotation. Curves A and B represent two conditions of adjustment of the same spring, which may be of either form illustrated. It will be noted that the spring is relatively insensitive to deflection below the critical setting. For example, and referring to curve B, the spring in question will accept a load of approximately 18 pounds and deflect only 0.004" but, upon an increase of only 2 pounds in the load, to 20 pounds, de-

' llection is rapid and continues as a substantial plateau.

By comparison, curve C is the linear plot of a conventional coil spring of uniform diameter and displays uniformly increasing deflection for uniformly increasing load. Stated otherwise, assuming actuation of the switch from closed to open upon a stroke of 0.010" of the button such event could occur at a load of approximately 3 pounds for a cylindrical coil spring as compared to approximately twenty pounds for a spring in accordance with the invention. Since ordinary fluctuations in torque below the overload torque may fall in a range corresponding to a load of 4 pounds it will be seen that the present invention provides a definite cut-off or knee, whereas the prior construction was absent this important advantage.

While we have shown certain embodiments of our invention, it will be understood, of course, that we do not wish to be limited thereto since many modifications may be made and we therefore contemplate by the pended claim to cover any such modifications as fall within the true spirit and scope of our invention.

We claim:

A non-linear torque overload protector for an electric motor-driven actuator comprising a J-shaped leaf spring with one leg substantially longer than the other, tension means urging said legs together to provide preload on said spring corresponding to a predetermined desired limit of load on said actuator, means applying load to said spring in parallel to said preload and in proportion to actual load on said actuator, and a switch actuated by displacement of said spring when said applied load exceeds said preload and operative to open electrical circuitry controlling said motor when actuated.

References Cited in the file of this patent UNITED STATES PATENTS Heikes Apr. 17, 1945 

